Manufacturing a finished container directly from a preform in a simple blow-molding machine and likewise manufacturing an intermediate container from a preform, or alternatively manufacturing a finished container from an intermediate container in a machine involving two blowing steps entails heating the body of the preform or of the intermediate container in a thermal conditioning unit, or oven, in which the material of which the body of the preform or of the intermediate container is made is heated by infrared lamps to raise it to a temperature above its glass transition temperature without, however, reaching its crystallization temperature. It is at the end of this phase during which its body is thermally conditioned that the preform or the intermediate container is transferred to a mold of a blow-molding or stretch-blow-molding unit in order to mold the finished or intermediate container by deforming the softened body.
The neck of this type of hollow body is always produced, when the preform is being manufactured by injection molding, at the definitive shape and size of the neck of the finished container and must not be deformed during subsequent treatment leading to the finished container; in particular, the neck must not be heated in such a way that it can soften. For this reason, the thermal conditioning unit is designed so that the neck of the preform of the intermediate container is protected from the heating elements as well as possible.
In on-line processing plants, the heat treatment is performed in tunnel ovens through which the hollow bodies (preforms or intermediate containers) are moved, by endless conveyer devices equipped with gripper devices capable of gripping them by their neck, the gripper devices also being mounted such that they can rotate on respective conveyer elements that make up the conveyer device so that the actual bodies of the hollow bodies, revolving on themselves, are exposed on all of their periphery to the heating lamps which are positioned down one side of the oven. In order to protect the necks during the journey through the ovens, it is known practice for the ovens to be equipped with protective shields positioned on each side of the path followed by the hollow bodies just at the boundary between their neck and their body. These shields form a screen and any radiation emitted by the heating lamps cannot reach the necks of the preforms or the intermediate containers directly.
However, because the radiation emitted by the lamps travels in all directions, some of the radiation may reach the necks from the inside of the hollow bodies after having entered these.
Nonetheless, in a common arrangement of the conveyer devices, the gripper devices for gripping the necks are configured in such a way as to be inserted inside the necks (a technique known as “donning” from the inside) and to hold them by clamping against the interior face of said necks. In this case, it is the heads of the gripper devices introduced into the necks which, by acting like a stopper, prevent any radiation that has entered the hollow bodies from reaching their necks.
For certain applications, however, it has become advantageous for the hollow bodies no longer to be grasped by the internal face of the neck, for example with a view to avoiding any bacteriological contamination of this face which will later have to be in contact with the product with which the finished container will be filled. That also makes it possible to eliminate any risk of scratching the internal face of the neck. In such cases, a gripper device that grips the hollow body from the external face of the neck (using the technique of “donning” from the outside) is then used.
Document FR 2 789 932 discloses a device for conveying hollow bodies, here consisting of preforms, equipped with gripper devices designed for donning from the outside and each comprising gripper claws which grasp an external face of the neck of the preform. The gripper device also comprises a central core which penetrates the inside of the neck axially and has a reflective lower transverse face which, when the preform is in place on the gripper device, lies axially more or less at the boundary between the neck and body of the preform.
More specifically, the gripper device comprises a gripper bell which has a circular top plate and a cylindrical skirt extending axially downward from the peripheral edge of the plate, so that the bell thus defines a cylindrical space that is open at the bottom. The bell has slits angularly distributed about its axis of revolution; each slit extends axially over the entire height of the skirt and also extends radially over approximately the outermost third of the plate. Thus, delimited between two slits is an elastically deformable claw, of which the free end, formed by the lower edge of the skirt, can move radially with respect to the axis of revolution of the bell. The bell thus designed is able to fit over the neck of the preform, the claws elastically grasping the external face of the neck. The internal face of the free end of the claws is smooth, which means the preform is grasped and held by simple radial clamping. The bell is made of a plastic material of the polyacetal type. To enhance the clamping force, an annular spring may encircle the bell at the lower edge of the skirt to force the free end of the claws radially inward. In the example proposed, the spring is housed in a circular groove formed on the external face of the skirt, at its lower edge.
The gripper device described in document FR 2 789 932 is mounted with rotation about its axis on a conveyer element that makes up the conveyer device.
The applicant company is assuming that the gripper device described in document FR 2 789 932 may be responsible for incidents such as the breakage of infrared heating lamps, and for problems of synchronization with the transfer means that affect the transfer between the oven that the hot preforms leave and the blow-molding unit. These problems are thought to be due to poor positioning of the neck of certain preforms in the gripper device. There are a great many parameters that can affect how well the preforms are gripped; for example: variation in dimensions of preforms, poor positioning of components on the preform conveyer line, thermal expansion associated with temporarily insufficient cooling of the gripper means, vibration, and thermal and/or mechanical fatigue.
The applicant company would therefore like to modify the structure of the gripper means in order as far as possible to reduce the number and frequency of the abovementioned incidents.
None of the fixtures of the prior art known to the applicant company seems able to reduce the frequency of these incidents, and some of these anterior devices are liable to cause other incidents.
Document US 2004/0121038 discloses a gripper device designed for the donning from the outside of a neck of a hollow body such as a preform. This gripper device comprises a bell-shaped component, the lateral wall of which is equipped with radial housings, distributed peripherally, opening to the inside via a narrowed opening; each housing houses a ball which is pushed back through the opening by an elastic member in such a way that the ball projects partially from the internal wall of the bell so that it can bear elastically under a relief on the external face of the neck of a preform pushed into the bell. In the example illustrated in FIG. 8 of that document, the balls bear against the edge of an annular groove made at the base of the neck of the preform and adjacent to the flange of the neck, the external edge of the bell bearing against the flange. This arrangement has the disadvantage of requiring the bell to be pushed a great way on to the neck; what is more, there is not always an annular groove at the base of the neck of the preforms.
More rapid donning could be obtained by pushing the bell on to the neck by a smaller amount, so that the balls could bear against reliefs formed by the screw thread designed for screwing a cap on to the finished and filled container. This type of donning could be suitable for all types of neck, including those, which have no annular groove at their base. However, in this case, because the screw thread is inclined, the balls would not all find identical bearing surfaces that converged radially in one and the same plane, which means that a torque would arise liable to cause the hollow body to become skewed, and the disadvantages set out above would then be experienced once again with the risk of the hollow body (the preform or the intermediate container) knocking against heating lamps and the risk of nonuniform heating of this object as it moves along, rotating, in its passage through the oven. In addition, the configuration of the screw thread on the neck of the hollow body may differ with different hollow bodies (single flight or portions of flights, different pitch and therefore different angles of inclination of the flight or flights, etc.): this would therefore cause the way in which the balls bear against the screw thread to vary according to the hollow bodies being handled.
It is a primary objective of the invention to propose an improved structure of a device for gripping hollow bodies by the neck which sets aside the above-mentioned disadvantages of the devices of the prior art and which is better able to meet the requirements of current practice, while at the same time remaining robust and reliable in design but simple and inexpensive.
The invention also aims to provide a device for gripping hollow bodies by the neck which is very able to tolerate dimensional variations in the diameter of the neck, and can quickly be adapted to suit different neck external contours.
The invention also aims to provide a device for gripping hollow bodies by the neck which takes up exactly the same amount of space as the devices that already exist on conveyer devices currently in use in order, in particular, that the anterior devices can be replaced, without any problems, with devices according to the invention.